Means for reducing audible noise developed by an extra high voltage transmission line

ABSTRACT

For reducing the audible noise developed by an extra-high voltage transmission line during foul weather conditions, the line conductor is covered with a coating of insulating material along generally that entire portion of its length which is located in a region where it is desired to limit audible noise. The coating has a thickness sufficient to withstand momentarily without puncture at least 80 percent of the line-to-ground voltage under conditions that initiate corona discharges from the external surface of the coating.

United States Patent [191 Liao [451 Apr. 23, 1974 MEANS FOR REDUCING AUDIBLE NOISE DEVELOPED BY AN EXTRA HIGH VOLTAGE TRANSMISSION LINE [75] Inventor: Tseng Wu Liao, Media, Pa.

[73] Assignee: General Electric Company,

Philadelphia, Pa.

[22] Filed: Jan. 10, 1973 [21] Appl. No.: 322,428

[52] US. Cl. 174/40 R, 174/127 [51] Int. Cl H02g 7/00 [58] Field of Search 174/40 R, 120 SC, 127,

[56] References Cited UNITED STATES PATENTS 1,626,777 5/1927 Austin 174/127 1,791,402 2/1931 Crowdes 3,096,210 7/1963 Boonstra 174/127 UX 3,641,251 2/1972 Liao 174/40 R FOREIGN PATENTS OR APPLICATIONS 1,092,809 11/1967 Great Britain 174/127 Primary Examiner-Laramie E. Askin Attorney, Agent, or Firm-William Freedman; J.

Wesley Haubner [5 7] ABSTRACT For reducing the audible noise developed by an extrahigh voltage transmission line during foul weather conditions, the line conductor is covered with a coating of insulating material along generally that entire portion of its length which is located in a region where it is desired to limit audible noise. The coating has a thickness sufficient to withstand momentarily without puncture at least 80 percent of the line-to-ground voltage under conditions that initiate corona discharges from the external surface of the coating.

6 Claims, 4 Drawing Figures MEANS FOR REDUCING AUDIBLE NOISE DEVELOPED BY AN EXTRA HIGH VOLTAGE TRANSMISSION LINE BACKGROUND This invention relates to an outdoor, overhead, a-c transmission line for voltages of 345 KV and higher and, more particularly, relates to means for reducing the audible noise developed thereby during foul weather conditions. Another approach to this problem is disclosed and claimed in my US. Pat. No. 3,641,251 assigned to the assignee of the present invention.

The typical transmission line in this voltage class includes a bare conductor at high voltage and a large air space around the conductor for providing the necessary insulation between the conductor and ground. During foul weather conditions, such as fog, rain or snow, such transmission lines can develop an objectionable amount of audible noise.

For reducing the audible noise developed by such transmission lines under foul weather conditions, it has been proposed to apply a relatively thin coating of insulation to the conductor or conductors of the transmission line. A typical thickness of such coating is to 30 mils. The rationale behind applying this thin coating was to change the surface effect at the highly stressed surface of the conductor in order to suppress coronatype discharges.

As a practical matter, however, this approach has been found to be relatively ineffective because the thin coating does not completely suppress corona, and the coating deteriorates relatively quickly in the presence of corona.

SUMMARY A general object of my invention is to reduce the audible noise resulting from corona discharge from a transmission line rated at 345 KV and higher.

Another object of my invention is to effect such audible noise reduction by reliable means which is highly resistant. to damage from the effects of corona dis-, charge. I

In carrying out my invention in one form, I provide an outdoor, overhead transmission line for voltages of 345 KV and higher that comprises an elongated metal conductor and a coating of insulating material covering the conductor along generally that entire portion of its length which is located in a region where it is desired to limit audible noise. The coated conductor is surrounded by air space that normally bears most of the line-to-ground voltage between said conductor and ground. The transmission line is subject to voltage and weather conditions that will initiate corona discharges from the external surfaceof the coating of insulating material. The coating has a thickness sufficient to withstand momentarily without puncture at least 80 percent of the line-to-ground voltage under conditions that initiate said corona discharges, each discharge being only about a microsecond in duration.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, referencemay be had to the following description taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view of a portion of a transmission line embodying one form of my invention.

FIG. 2 is an enlarged sectional view along the line 2-2 of FIG. 1.

FIG. 3 is a side view of the coated conductor depicted in FIG. 2.

FIG. 4 is a side elevational view showing a modified form of transmission line.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown an alternating current transmission line for outdoor, overhead circuits rated for voltages of 345 KV and higher. Such voltages are referred to herein as extra high voltages (EHV) and ultra high voltages (UI-IV). EHV comprehends voltages from about 345 KV to about 765 KV and UI-IV comprehends higher voltages.

This transmission line of FIG. 1 comprises a conductor assembly 10 (soon to be described) and a suspension insulator 7 that supports the conductor assembly from the arm 8 of a suitable conductor-supporting tower. It is to be understood that other insulators and towers (not shown) of a similar construction are provided at points spaced apart along the length of the conductor assembly for supporting the assembly at these points.

Referring now to FIGS. 2 and 3, the conductor assembly 10 comprises a conductor 11 of the type commonly referred to as ACSR. Such conductor comprises a plurality of aluminum strands 12 on its outer periphery helically wound about a stranded steel core 14. Surrounding the conductor 11 is a thick coating, or covering, 16 of a suitable electrical insulating material that is capable of withstanding long-term exposure to outdoor weather conditions without significant chemical or mechanical deterioration. An example of such an insulating material is the polyolefin resin sold by General Electric Company under the trademark Vulkene. Although a portion of the insulating covering 16 is shown broken away in FIG. 3, it is to be understood that the insulating covering 16 covers the conductor 11 along essentially that entire portion of its length which is located in the region where it is desired to limit audible noise. Typically, in such a region, the insulating cover 16 will be unbroken over the entire span between spaced-apart suspension insulators 7, which is ordinarily many hundreds of feet.

In presently-existing EI-IV transmission lines, the high voltage conductor is usually bare. Large clearance spaces are provided between the high voltage conductor and ground, and, in almost every case, the intervening electrical insulation is constituted entirely by the air that surrounds the bare conductor. During foul weather conditions, such as fog, rain or snow, corona discharges usually develop from the conductor; and such discharges are objectionable, among other reasons, because they result in audible noise.

One way of reducing the audible noise level is to apply to the conductor a thin coating of electrical insulation which provides a surface effect acting to cover the metal surface irregularities and therefore decrease the voltage gradient at the surface of the conductor,-

thereby suppressing the corona discharges responsible for the audible noise. But this approach has proven to be not very practical because it does not entirely prevent the development of corona, and the thin coating deteriorates rather quickly in the presence of corona.

In studying this problem, I have observed that under simulated foul weather conditions corona discharges often develop from the external surface of the wet insulating coating. During the short discharge period, current flows from the conductor through the series combination of the insulation and the corona discharge, developing a voltage drop across the insulation equal to the product of the insulation resistance and the corona current. This voltage drop may be momentarily as high as 80 percent of the line-to-ground voltage. If the insulation cannot withstand this high momentary voltage, it will be punctured at the site of the discharge in the form of a pinhole. Then the next discharge at or near the pinhole will lead to a heavy discharge from the conductor since there is no longer any high insulation resistance available to limit the current. This heavy discharge will destroy a substantial amount of the insulation, greatly enlarging the original pinhole, thus leaving the wire effectively bare in this region and with its original susceptibility to corona discharge.

Instead of the thin coating previously proposed, I use a very thick coating that can momentarily withstand without puncture at least 80 percent of line-to-ground voltage under the weather and voltage conditions that initiate corona discharges from the external surface of the insulation. This thick coating provides a very high resistance (many megohms) for limiting the magnitude and duration of the discharge current. This reduces the energy of the discharge sufficiently to assure against damage to the insulation.

Another factor which reduces the energy of the discharge is that, with the thick insulation, only the capacitance at the site of the discharge discharges its energy instead of the capacitance of a long length of the transmission line, as with the thin insulation.

In referring to thick insulation, I am referring to insulation having a thickness of at least about 250 mils, as compared to the to 30 mils present in the thin insulation approach. The minimum thickness will depend upon the particular insulating material being used and the particular voltages being dealt with, but in any case this minimum thickness should be high enough to with stand without puncture at least 80 percent of the lineto-ground voltage during momentary corona discharges.

It is to be understood that my main purpose in using the thick insulation is not to change surface effects at the conductor surface but rather to reduce the intensity and duration of the energy discharge (by inserting a high impedance between the conductor and the corona discharge). By so reducing the intensity and duration of the discharge, I reduce the audible noise without allowing damage to the insulation.

It should be understood that when there is no corona discharge in the adjacent air space outside the insulating coating, which is the case under normal conditions, the voltage drop across the insulating coating is very small, e.g., considerably less than 5 percent of the lineto-ground voltage. Thus, under normal conditions, there is very little voltage stress on the insulation, and there is little or no likelihood during this time of its being punctured by voltage stresses.

As will be evident from the immediately-preceding paragraph, in the type of electrical equipment that I am concerned with here, the primary electrical insulation is the large air space around the high-potential conductor. This is a very different class of electrical equipment from that in which a thick insulating coating is provided to withstand the full line-to-ground voltage that is normally present. My present invention is not concerned with this latter class of electrical equipment.

It should also be noted that I am concerned with a class of electrical equipment where the operating voltage is so high that it can develop corona in the adjacent air space outside any electrical insulation on the conductor (under foul weather conditions). My invention is to be distinguished from the type of equipment where this phenomena is not present and every effort is made to avoid corona under all conditions of operation.

My invention is also to be distinguished from the type of electrical equipment that comprises a conductor mounted within a grounded metal tube and separated from the tube by a gap of several inches, or possibly as much as about a foot, containing compressed gas insulation. To the conductor of such equipment it has been proposed to apply a relatively thick insulating coating (as disclosed, for example, in U.S. Pat. No. 3,515,909- Trump); but the purpose of such coating is not to suppress audible noise but rather to prevent an electrical breakdown between the conductor and the grounded metal tube. I am dealing, however, with a transmission line where the stressed gap between the conductor and earth (except at the very limited regions where at tached to support towers) is so great in length (e.g., more than 30 feet) that the large air space alone is sufficient to prevent a breakdown thereacross even under the most severe transient or impulse conditions. This latter characteristic of an outdoor transmission line makes it unnecessary for my solid insulating coating to be able to withstand the full line-to-ground impulse voltage. Even if a puncture of the solid insulation coating should occur in the region at the support tower, this would not have a significant effect on the overall audible noise level because this region is of only a very small extent compared to the hundreds of feet of conductor extending between adjacent support towers.

To reduce the chance for any corona developing between the conductor 11 and the insulating cover 16, the insulating cover has a thin layer of conductive or semiconductive material integral with its internal sur face and contacting the conductor 11. Thus, if any void should be present between the outer periphery of conductor l1 and the inner periphery of insulating cover 16, there is little chance of corona developing across this void because the surfaces on opposite sides thereof are at the same potential. It should also be noted in this connection that any such void would normally be under very little voltage stress since, as pointed out above, only a few percent of the total line-to-ground voltage normally appears between the conductor 11 and the outer surface of the insulating cover 16.

My invention in its broader aspects is also applicable to the type of transmission line that includes one or more shielding wires located adjacent the main conductor or conductors. The purpose of these shielding wires is to reduce the surface voltage gradient adjacent the main conductor or conductors. Such a transmission line is illustrated in FIG. 4, where there are two main current-carrying conductors 30 and 31 connected electrically in parallel and two shielding wires 32 and 34 spaced from the current-carrying conductors. Suitable spacing means of insulating material, schematically shown at 36, is provided for maintaining a generally fixed spacing between these conductors. Each of the main current-carrying conductors 30 and 31 is bare. Each of the shielding wires 32 and 34 is surrounded by a thick coating of insulating material which serves the same purpose as the coating 16 in FIG. 1, that is, to reduce the intensity and duration of the discharge which accompanies the development of corona at any location along the conductor, thus reducing the resulting audible noise.

It is to be understood that the shielding wires are suitably electrically connected to the main conductors at spaced points along their length in order to maintain physically adjacent regions of all the wires at substantially the same potential.

The shielding wires 32 and 34 are of a relatively high resistivity material compared with that of the main'conductors and therefore carry an almost negligible amount of current. In view of this low current, only a relatively small amount of heat needs to be dissipated from these wires, and a thick insulating coating around such wires can be easily tolerated from a temperaturerise viewpoint.

While I have shown and described particular embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from my invention in its broader aspects; and I, therefore, intend herein to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by letters Patent of the United States is:

1. In an outdoor, overhead, alternating-current transmission line for voltages of 345 KV and higher, extending through a region where it is desired to limit audible noise from the transmission line,

a. an elongated metal conductor and a coating of insulating material covering said conductor with no substantial space between said coating and the periphery of said conductor along generally that entire portion of its length extending through said region,

b. said coated conductor being suspended by insulators and surrounded by air space that normally bears most of the line-to-ground voltage between said conductor and ground, said air space providing an air gap under stress between said line and the surface of the earth greater than 30 feet in length,

0. the transmission line being subject to voltage and weather conditions that will initiate corona discharges from the external surface of the coating of insulating material,

d. said coating having a thickness sufficient to withstand momentarily without puncture at least percent of the line-to-ground voltage under conditions that initiate said corona discharges but a thickness insufficient to prevent said corona discharges.

2. The transmission line of claim 1 in which said elongated metal conductor is a shielding wire and in which there is another metal conductor that serves as a main current-carrying conductor of the transmission line, the shielding wire running alongside said other conductor and having substantially the same potential at points along its length as adjacent portions of said other conductor.

3. The transmission line ofclaim 2 in which said main current-carrying conductor is essentially bare along most of its length.

4. The transmission line of claim 1 in which some of said suspension insulators are provided at points spaced apart by at least several hundred feet along the length of said conductor for supporting said conductor, said insulating coating covering said conductor along substantially its entire length between said latter insulators.

feet of its length. 

1. In an outdoor, overhead, alternating-current transmission line for voltages of 345 KV and higher, extending through a region where it is desired to limit audible noise from the transmission line, a. an elongated metal conductor and a coating of insulating material covering said conductor with no substantial space between said coating and the periphery of said conductor along generally that entire portion of its length extending through said region, b. said coated conductor being suspended by insulators and surrounded by air space that normally bears most of the lineto-ground voltage between said conductor and ground, said air space providing an air gap under stress between said line and the surface of the earth greater than 30 feet in length, c. the transmission line being subject to voltage and weather conditions that will initiate corona discharges from the external surface of the coating of insulating material, d. said coating having a thickness sufficient to withstand momentarily without puncture at least 80 percent of the lineto-ground voltage under conditions that initiate said corona discharges but a thickness insufficient to prevent said corona discharges.
 2. The transmission line of claim 1 in which said elongated metal conductor is a shielding wire and in which there is another metal conductor that serves as a main current-carrying conductor of the transmission line, the shielding wire running alongside said other conductor and having substantially the same potential at points along its length as adjacent portions of said other conductor.
 3. The transmission line of claim 2 in which said main current-carrying conductor is essentially bare along most of its length.
 4. The transmission line of claim 1 in which some of said suspension insulators are provided at points spaced apart by at least several hundred feet along the length of said conductor for supporting said conductor, said insulating coating covering said conductor along substantially its entire length between said latter insulators.
 5. The transmission line of claim 1 in which said coating of insulating material is provided with an inner periphery that contacts said conductor and is of a conduction or semiconductive material.
 6. The transmission line of claim 1 in which said coating covers said conductor over at least many hundred feet of its length. 